Factor GUIDMixin definition in a new file.

* Abstract initialization of GUIDMixin inside
  initialization of CryptoPeerConnection
* Coerce string to GUIDMixin when adding contact.
Support unicode GUIDs.

* Add a deprecation comment for future comparisons
  against subclasses of basestring.
Test GUIDMixin.

* Also test CryptoPeerConnection subclassing.
Update documentation.
Merge pull request #693 from Renelvon/guid_is_unicode

Abstract GUIDs. Properly support unicode GUIDs.
Flag implicit relative imports (W0403).
State relative imports explicitly.

* Reorder imports in accordance to PEP8.
* Replace calls to privtopub with calls to
  privkey_to_pubkey for consistency.